Sonic Refrigerators
Some refrigerators keep cool by using sound emitted by loudspeakers. Scientists at Los Alamos National Laboratory made such a refrigerator and named it STAR (Space ThermoAcoustic Refrigerator) in the early 1980’s. STAR was first used to cool electronics aboard the space shuttle Discovery in January 1992. Within the next five years, refrigerators and freezers whose design is based on STAR may be used in our homes.
To understand how this new breed of sonic refrigerators works, we recall that around 1800, Joseph Gay-Lussac observed that for a sample of gas in a container of constant volume, the pressure exerted by the gas is directly proportional to the absolute temperature. He also found that the rate of increase in pressure with increasing temperature was independent of the gas studied. Thus, doubling the absolute temperature of a sample of gas also doubles its pressure at constant volume.
The sonic refrigerator uses sound waves from two four-inch speakers to exert pressure on a mixture of argon and helium confined in a specially-shaped container. Because gases warm as they are compressed and cool as they expand, the mixture of gases heats up as the sound exerts a pressure on the gases, then cools down when the pressure decreases in the absence of sound. This sound is typically between 10,000 and 100,000 times louder than that of a typical rock concert, but the sound is contained within rigid, nonvibrating plastic walls. As long as the walls don’t vibrate, no sound escapes from the refrigerator. In fact, the unit is so quiet that sometimes a stethoscope is used to check if the unit is operating properly.
The speakers create a standing wave of sound in the gas. This means that the gas in a certain region of space is always compressed (and thus warmer than the average gas) and the gas in other regions is always expanded (and cooler than average). Near the speaker the pressure is high, as is the temperature. A heat exchanger, similar to a car’s radiator, removes excess heat from the pressurized gas sample. Half a wavelength from the speaker the pressure and the temperature is low. Liquid in a surrounding chamber is then run through the cooled gas to lower the temperature of the liquid; the cool liquid is then circulated around a storage compartment that contains food. For pure He at room temperature and at a frequency of 550 Hz, the wavelength of sound is 1.8 meters, making half the wavelength 0.9 meters. The cold gas is then 0.9 meters away from the hot area, quite a reasonable distance. The device opens up shortly after the cold area.
Efforts are currently being directed at developing a thermoacoustic refrigerator for home use. Many environmentalists are excited about this because this type of refrigerator would not use chlorofluorocarbons (CFCs), one of the principal contributors to the destruction of the ozone layer. The principles of the thermoacoustic refrigerator may also be utilized in a new generation of air conditioners.
Whitten/Davis/Peck: General Chemistry with Qualitative Analysis, 5/e, p. 402
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